Electrical arcs have been widely employed for fuel ignition in combustion devices, a typical example being the use of spark plugs in internal combustion engines. Spark plugs generate a short duration electrical arc across the electrodes by impressing a very high voltage potential between the electrodes, i.e., 25,000-50,000 volts or higher, which leads to an initial electrical discharge across the gap at relatively high current levels on the order of several hundred amperes.
There has also been employed so called "make and break" spark igniters in which relatively movable electrodes are in contact and momentarily separated to create a spark between the electrodes.
Such ignition systems have not in recent times been employed for engine ignition in favor of the fixed gap spark plug ignition systems due to excessive erosion of the electrodes exposed to the combustion process.
U.S. Pat. No. 4,757,788 describes a recent attempt to apply the "make and break" ignition systems in an internal combustion engine.
Electrical arcs have also been employed as heaters, in which an electrical arc is sustained while a gas is circulated over the arc to heat the gas rapidly to high temperatures. Such heaters use a vertical flow of gas to stabilize the arc, causing it to rotate around cavities within opposing electrodes to alleviate the severe electrode erosion problem which would otherwise be encountered by a sustained electrical arc heating the electrode surface.
In U.S. Pat. No. 4,672,938, assigned to the same assignee as the present application, there is described an "activation" of fuel by the formation of molecular radicals in the fuel prior to combustion, so as to achieve "hypergolic" combustion, i.e., combustion with only a negligible delay after introduction of the fuel into an oxidizing atmosphere. Such activation may be accomplished by an electrical arc, and indeed may be the mechanism whereby spark ignition normally occurs.
In copending application Ser. No. 290,976, now U.S. Pat. No. 4,865,003, by the present inventors, there is disclosed the activation of fuel preparatory to combustion by exposing the fuel to an arc sustained for a period sufficient to activate a substantial proportion of the fuel charge burned, as in an internal combustion engine. While for this situation an arc need only be sustained for a relatively brief period compared to arc heaters, its duration is sufficient to necessitate solving the electrode erosion problem. Maintenance problems, even if only moderate, desirably should be avoided if possible. Thus, the arc should only be sustained for the period necessary for activation of each fuel charge, but it is desirable to maximize the exposure of the fuel charge to the electrical arc to enhance the activating effect of the arc on the fuel charge.
It is an object of the present invention to provide electrode arrangements for establishing a sustained arc to which a charge of fuel may be exposed so as to activate a substantial proportion of the charge, and which does not require a high voltage to initiate nor result in excessive electrode erosion.